Time of flight imaging is used in a number of applications including range finding, depth profiling, 3D imaging (e.g. LIDAR) and medical imaging techniques. Direct time of flight measurement comprises directly measuring the length of time between emitting radiation and sensing the radiation after reflection from an object. From this the distance to the object can be determined. In specific applications, the sensing of the reflected radiation may be performed using a Single Photon Avalanche Diode (SPAD) array. SPAD arrays have been used as solid-state detectors in imaging applications where high sensitivity and timing resolution are required.
A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionization. The avalanche is quenched, either actively or passively to allow the device to be “reset” to detect further photons. The initiating charge carrier can be photo-electrically generated by means of a single incident photon striking the high field region. It is this feature which gives rise to the name ‘Single Photon Avalanche Diode’. This single photon detection mode of operation is often referred to as ‘Geiger Mode’.
Time to digital converters are sometimes used in time of flight imaging applications to increase timing resolution over that of a single clock cycle. However, TDC circuits are presently only able to process one event in a single measurement cycle.
It would be desirable to provide a TDC that is able to process multiple events in a single measurement cycle.